6.8.2 Rating of Voltage Taps
Transformers should be supplied with taps to allow adjustment in bus voltage. Optimum tap settings can
be determined using a load flow study.
6.9 Motors
6.9.1 Selection of Motors
Many motors are required in a thermal generating plant and range in size from fractional horsepower to
several thousand horsepower. These motors may be supplied with the equipment they drive or they may
be specified by the electrical engineer and purchased separately. The small motors are usually supplied by
the equipment supplier and the large motors specified by the electrical engineer. How this will be
handled must be resolved very early in the project. The horsepower cut-off point for each voltage level
must be decided. The maximum plant voltage level must be established. A voltage of 13.8 kV may be
required if very large horsepower motors are to be used. This must be established very early in the plant
design so that a preliminary one-line diagram may be developed.
6.9.2 Types of Motors
6.9.2.1 Squirrel Cage Induction Motors
The squirrel cage induction motor is the most common type of large motor used in a thermal generating
plant. Squirrel cage induction motors are very rugged and require very little maintenance.
6.9.2.2 Wound Rotor Induction Motors
The wound rotor induction motor has a rotor winding which is brought out of the motor through slip
rings and brushes. While more flexible than a squire cage induction motor, the slip rings and brushes
are an additional maintenance item. Wound rotor motors are only used in special applications in a
power plant.
6.9.2.3 Synchronous Motors
Synchronous motors may be required in some applications. Large slow-speed, 1800 rpm or less may
require a synchronous motor. A synchronous motor may used to supply VARs and improve voltage
regulation. If the synchronous motor is going to be used as a VAR source, the field supply must be sized
large enough to over-excite the field.
6.9.2.4 Direct Current Motors
Direct current motors are used primarily on emergency systems such as turbine lube oil and turbine
turning gear. Direct current motors may also be used on some control valves.
6.9.2.5 Single-Phase Motors
Single-phase motors are fractional horsepower motors and are usually supplied with the equipment.
6.9.2.6 Motor Starting Limitations
The starting current for induction motors is about 6 times full load current. This must be taken into
account when sizing transformers and should be part of the load flow analysis. If the terminal voltage is
allowed to drop too low, below 80%, the motor will stall. Methods of reduced voltage starting are
available, but should be avoided if possible. The most reliable designs are the simplest.
6.10 Main Generator
The turbine generator will be supplied as a unit. The size and characteristics are usually determined by
the system planners as a result of system load requirements and system stability requirements.